Limitations on fastening cold-formed metal thicker to thinner using tek screws?

[abusementpark]

Are there limitations on fastening thicker cold-formed metal to thinner cold-formed metal using Tek screws (that is, the self-drilling is performed on the side of thicker material)?

For example, can you self-drill a 12 gauge bent plate into a 26 gauge floor deck where the screw is installed through 12 gauge plate first?

 

[jayrod12]

What would lead you to a situation where you were required to do something like that? What kind of load would be travelling through a 12 gauge bent plate that a couple of teks to 26 gauge steel deck is sufficient?

 

[JAE]

Normally you want to screw from the thinner side and that is what the testing, load tables, etc. are all based on.

If you go the other direction, I think you'd have to pre-drill holes through the thicker part first. But then you'd still have a condition that hasn't been tested for load capacity.

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[MotorCity]

It also depends which side the load is on and determine whether a pull-out, pull through, or both limit states apply

 

[azcats]

I just skimmed thru the ESR report and I didn't see such a limitation. Table 2 specifically covers the situation. Loads are small as you'd expect but it doesn't seem to be prohibited.
Loa
[URL unfurl="true"]https://www.icc-es.org/wp-content/uploads/report-directory/ESR-1976.pdf[/url]

For ease of installation, it might be worth pre-drilling/pilot thru the 12 gauge plate (or both). I have vague recollections of a contractor complaining that it took more than one screw to get drilled thru some thick material, but it was likely heaver than 12 ga. Not sure if this is a one off or there are hundreds of these.

 

[XR250]

I agree with JAE - You would have to pre-drill the 12ga plate or else it will never pull the plys tight.

 

[NorthCivil]

I practice quite a lot of engineering of little screws and bits.

screws in pure tension fail in 3 ways: pull out, pull-over, and fastener failure (usually the head popping off)

I cant recall ever having to screw through the thick substrate into the thin, but if such a situation demands it, I agree with Jay, first drill a passhole in the thicker substrate.

though I will caution you on screwing into a 26 guage deck - at that guage there will be no meaningful engagement of the thread.

I generally wont rely on a substrate to support a screw unless it is min 2mm thick, and even then, as with nails, little fasteners are much more reliable and effective in groups

 

[dhengr]

Abusementpark:
There is a limit to how thick material can be when using self-tapping screws. You just plain start to wear the drilling tip out before you get through the thicker material, particularly when it is a high strength material. Most of those tips are just lightly case hardened, not really a drill tip. The ESR report should comment on these limits. I think it is usually around 3/16” or 1/4” thick material for most self-tapping screws.

The reasons you don’t usually drill through the thicker material and into the thinner material, is partly tip wear, and the fact that you may not even pierce the thinner material, and not really even know it. You have to provide some backing for the thinner material during screwing to be sure it just doesn’t flex out of the way under a dull tip. Secondly, the torque effort (drilling effort) through the thicker material makes the worker less sensitive to what’s happening in the thinner material. And, there is a high likelihood of just stripping those thinner threads. I, too, would predrill the thicker part first.

 

[wannabeSE]

I’ve never seen any definitive guidance. Often times, I’ll add a note to pre-drill as required and let the contractor decide. For, the example through 12 gage into 26, I’d add a note to pre-drill without the “as reqd” If it were something truly structural (eg supporting a floor), I wouldn’t be as cavalier

 

[abusementpark]

jayrod 12 - I gave an extreme example, but this situation comes up a lot in various conditions. A good example would be the connection of a low roof metal deck to an adjacent CMU wall that supports a high roof. Let's say I am using a 24 gauge metal roof deck and this deck needs to act as as a diaphragm to transfer in-plane shear parallel to the wall as well out-of-plane tension or compression perpendicular to the wall. I would typically provide a bent plate to attach the deck to the wall. The bent plate would get installed after the roof decking is installed. That bent plate would also need to be thicker material than the roof deck since it will be subject to eccentricity and flexural stresses. So, maybe I need it to be a 12 gage bent plate and the fastening has to occur from above. Thus, you would be fastening through the thicker material into the thinner material.

azcats - Table 2 in that ESR report covers a pull-out failure for the material not in contact with the screw head (e.g. the top chord of a cold-formed metal roof truss that has roof deck fastened down to it), but I don't think that specifically addresses a thicker material being fastened to thinner material.

I agree with you guys that pre-drilling is likely necessary in the situation I described (12 GA to 26 GA), but what is the limit? Does 20 GA into 16 GA need pre-drilling or is the difference close enough? And if pre-drilling is required, what diameter pre-drill should be used on the thicker material?

 

[azcats]

Sorry - I intended to mention Table 4 (not table 2) where they specifically note shear values for thicker to thinner and thinner to thicker.

 

[abusementpark]

Azcats - thanks for the clarification, that is a good reference.

I emailed ITW Buildex asking about the pre-drilling and will report back if/when they respond.

 

[EdStainless]

I have encountered this when building some machine enclosures.
We predrilled the heavy material and then also backed the thin material with a strip of intermediate gage.
This assured that we got some bite, and at least clamped the edge of the light material.

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P.E. Metallurgy, Plymouth Tube

 

[abusementpark]

Here's what I got from ITW Buildex:

"Thank you for your inquiry. The fastener has the capability to drill and tap through both materials but in order to pull the 22 gage tight against the 12ga. you should predrill the gage steel large enough for the fastener pass through and be able to drill and tap into the 22 gage. and pulling it tight against the 12 gage. Let us know if you need any additional information."

When I followed up about the size of the pre-drill:

"That depends of the size of the fastener you are using. See below.

Fastener size Clearance hole size

#10 = .190” 13/64 (.203”)
#12 = .210” 7/32” (.218”)
¼= .250” 17/64 (.265) "